1. Field of the Invention
The present invention relates to an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine.
More specifically, the present invention relates to an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine to a peripheral outer surface of the dryer.
2. Background Information
Paper is normally dried by passing it over a series of steam-heated, cast iron dryer cylinders. These cylinders are typically 4′, 5′, or 6′ in diameter, with some modern dryers being as large as 7′ in diameter. The steam inside the dryer cylinders transfers its heat to the paper through the dryer shell. As the heat is transferred from the hot steam to the wet paper, the steam inside the dryer condenses. The condensate thus formed is then removed from the dryer cylinder through a syphon pipe connected to an external pipe or tank through a rotating seal.
At low rotational speeds, the residual condensate inside the dryer will tend to accumulate in a puddle in the bottom of the dryer cylinder, in a “ponding” state. As the dryer rotational speed increases, the condensate in this puddle will begin to rotate with the dryer shell, and then fall back into the puddle. This is normally referred to as the “cascading” state. At high dryer speeds, the condensate will follow the dryer cylinder around the entire periphery of the dryer shell, in a state that is called “rimming”.
In order to minimize the power required to rotate the dryers in the ponding and cascading states, and to maximize the transfer of heat through the rimming condensate, the dryer syphons are normally designed to minimize the amount of condensate in the dryers.
At high speed, however, even thin residual layers of condensate can form a significant resistance to the transfer of heat from the steam to the dryer shell. At high speed, the rimming layer of condensate is very stagnant and forms an insulating barrier between the steam inside the rimming condensate layer and the inside surface of the dryer shell.
Dryer bars were developed to generate turbulence in the rimming layer, in order to increase the rate of convective heat transfer through the layer. Dryer bars consist of a series of solid metal bars that are located inside the dryer cylinder. The bars are held by various means against the inside surface of the dryer cylinder. The bars tend to generate turbulence in the riming layer of condensate that forms between the individual bars. This increase in condensate turbulence increases the rate of heat transfer and also tends to improve the uniformity of heat transfer from the dryer cylinder.
The concept of dryer bars was first disclosed by Barnscheidt and Staud in U.S. Pat. No. 3,217,426. Specific formulae for predicting the optimum amount of condensate was later added by Appel and Hong as taught in U.S. Pat. No. 3,724,094. Several methods have since been developed for holding these bars to the inside surface of the dryer. One method, for example, uses a series of magnets to hold the bars to the dryer shell surface as described in Mathews U.S. Pat. No. 4,195,417. Another method uses a series of bars that are magnetic as disclosed by Wedel in U.S. Pat. No. 4,486,962. Other methods have been disclosed by Kraus (U.S. Pat. No. 3,808,700), Schiel (U.S. Pat. No. 4,267,644), and Schiel (U.S. Pat. No. 4,282,656), using various types of springs and pins.
In each of these prior art arrangements, the bars have consisted of solid metal bars (normally mild steel, but sometimes stainless steel, for use in corrosive environments). Bars used in commercial embodiments have square or rectangular cross-sections, ranging from 0.25″×0.25″ to as large as 0.5″×0.75″. This cross-section is selected based on the number of rows of bars in the dryer, the amount of condensate that is expected to be rimming inside the dryer, the cost of the bars, the rigidity of the bars, and the ability to handle the bars during installation.
The weight of large cross-section bars makes their installation very difficult, particularly when installed inside existing papermaking dryers that are only 4–6′ in diameter. Bars with small cross-section are much easier to handle, but do not have the structural rigidity to withstand long periods of tumbling of condensate inside the dryer.
Most papermaking dryers have removable cast ports in the front (tending side) head. These ports (“manholes”) are removed to provide access for inspecting the inside of the dryer cylinders and for installing and maintaining syphon equipment. To avoid the very difficult task of removing the dryer heads, dryer bar equipment must fit through these manholes in order to be installed in existing dryer cylinders. This limits the design of the apparatus for holding the bars in place.
Further, modern papermaking machines produce paper up to 400″ in width, running at speeds approaching 6,000 feet per minute. These machines can produce over 1,000 tons of paper per day. The cost of having these machines idled for installation of dryer bars can be very high, often exceeding $15,000 per hour. A reduction in the time required to install dryer bars inside existing dryer cylinders can provide a very significant reduction in the idle time for the machine. Despite this incentive for short installation times, the time required to install prior art dryer bars is still typically 1.5–2.5 hours per drying cylinder. Prior art methods have not provided significant reductions to this installation time.
Most prior art bars are held against the dryer shell using a series of hoop segments. In order to hold the bars tightly against the dryer shell, these hoop segments must be pressed toward the shell surface. In the prior art designs, this force is developed by installing various systems between flanges at the end of the hoop segments, to force the segments apart.
One of these systems is a simple threaded turnbuckle with locking nuts. These turnbuckles are tightened using a pair of open-end wrenches. This is a time-consuming process. The rigid turnbuckles do not provide much resilience to allow for the differential thermal expansion of the dryer shell, with respect to the dryer bar hoops. Without a method for allowing for differential thermal expansion, the stress in the turnbuckle, the stress in the hoop segments, and the stress on the dryer shell will increase. This can cause deformation and long-term loosening of the hoop assembly.
A more sophisticated design uses various types of springs between the hoop segments. These springs have alternately been coil springs, cylindrical springs, and Bellville washers. These springs maintain the design force of the segments against the bars as the dryer is heated up, but the time required to install these systems is much longer. There are more parts to handle and additional hand tools are needed for their installation.
The prior art bars are attached to the hoop segments to prevent them from shifting in the circumferential direction. The bars are normally attached with small threaded fasteners (capscrews). These fasteners require some mechanism to lock them in place, so that they do not come loose inside the dryer cylinder. The locking mechanisms used in prior art dryers include split washers, Bellville washers, flanged self-locking fasteners (WhizLock), and groove lock pins. The threaded fasteners can be difficult to align during installation. It can be difficult to get the fastener started in the threaded holes in the bars, and self-tapping screws can be easily broken. Small diameter pins can be difficult to align, they are easy to break off, and they can come loose inside the dryer.
The present invention provides a method and apparatus for improving the drying capacity of steam-heated cylinders, and in particular cylindrical dryers in a papermaking machine, the apparatus utilizing a series of bars disposed in a generally axial direction inside and adjacent to the shell of the dryer cylinders. The invention more specifically provides for an apparatus which includes hollow rectangular bars, means for holding the bars against the dryer shell, and a method of installing the apparatus. The means for holding includes a fastening system for the bars. The fastening system includes, in combination, a series of hoop segments that are coupled together with special fasteners, a series of bars that are coupled to the hoop segments with special pins, and a unique bar geometry to reduce the time and effort required for their installation.
The dryer bars of the present invention provide a stiffer structure with a lighter weight than existing bar configurations. The apparatus of the present invention can reduce the installation time by approximately a factor of 3. The construction is low in cost and the bar geometry provides heat transfer that equals or exceeds that of the prior art dryer bar configurations.
In order to reduce the weight of the dryer bars, the bars of the present invention are hollow rectangular tubes. These tubes are much lower in weight with much higher bending stiffness than the prior art bars. This greatly improves the ease of handling the bars for installation and makes them less susceptible to bending when subjected to the impact forces of tumbling condensate.
For example, the weight of a typical 0.5×0.75″ solid steel cross-section dryer bar that is 6′ in length is 7.6 pounds. The installation crew must handle 138 pounds of steel bars to install a segment with 18 rows of bars. The weight of one of the bars of the present invention (preferred size is 0.75″×1.00″ with a 0.065″ wall thickness) is only 4.3 pounds and the installation crew must handle only 77 pounds during the installation of a similar segment with 18 rows of bars.
Also, the stiffness of the bars of the present invention is significantly increased. The moment of inertia of the prior art bars in the previous example would be 0.008 in4 in the radial direction and 0.018 in4 in the circumferential direction. By comparison, the moment of inertia of the bars of the present invention, for the preferred size, is be 0.018 in4 in the radial direction and 0.029 in4 in the circumferential direction, that is, 130% stiffer in the radial direction and 60% stiffer in the circumferential direction. All while being lighter in weight.
The bars of the present invention are held against the dryer shell using a series of hoop segments, as is done in most prior art configurations. In order to hold the bars tightly against the dryer shell, these hoop segments are pressed toward the shell surface with a unique threaded fastener. This fastener system consists of a threaded fastener and a threaded nut. The head of the fastener extends through a hole in the end of the hoop segment. This head holds the fastener in place during installation and during operation. The head of the fastener has a socket head. However, the head could alternatively have an external hex shaped configuration. This allows the fastener to be turned using either manual or automatic (electric or pneumatic) ratchets to tighten the fastener, pressing the threaded nut against the flange on the adjacent hoop segment. This greatly speeds up the installation process.
The bars of the present invention are held to the hoop segments using large-diameter pins. These pins are installed in the hoop segment prior to the installation of the bars. This eliminates the time required to find, start, and then engage conventional pins and threaded fasteners. These pins also have a shoulder that prevents them from coming out of the hoop segment, even after the segment has been in service for many years.
A portion of the normal differential thermal expansion between the dryer shell and the bar assembly is absorbed by the radial flexibility of the hollow rectangular tube bars. This, coupled with the flexibility of the hoop segments, allows the bar assembly to handle normal differential thermal expansion without the need for complex systems of springs or flexible hoop couplings.
Because the bars are lighter in weight for a given cross-section, the overall cross-section of the tube bars can be increased to values larger than would be practical with solid bars. This allows the selection of larger bars to optimize the generation of turbulence in the rimming condensate, to gain the maximum heat transfer.
The tube bars can also be manufactured economically in stainless steel, for dryers in which corrosion is a problem. The high cost of stainless steel normally precludes the use of stainless with solid dryer bars, except for very special applications where the high cost would be acceptable. With the lower cross-sectional area of material, stainless steel can be used in place of mild steel while retaining costs that are competitive with respect to solid mild steel bars.
Therefore, it is a primary feature of the present invention to provide an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine that offers improvemements in performance over that provided by the prior art arrangements.
Another feature of the present invention is the provision of an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine that is relatively easy to manufacture.
A further feature of the present invention is the provision of an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine that is of relatively low cost.
Another feature of the present invention is the provision of an apparatus for increasing a transfer of thermal energy through an inner surface of a hollow cylindrical dryer of a papermaking machine that is very easy to install.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.